The present invention relates to a method for heat transfer such as cooling method for a semiconductor device, and more particularly, to a cooling method of using a metal sherbet, consisting of metal being in the state of a mixed phase or two-phase mixture of a liquid phase and a solid phase, as a heat conducting body disposed between a cooling unit and the heat source for transferring heat generated by the device to the cooling unit in high cooling efficiency.
Recently, to improve processing ability of an information processing system, respective sizes of the transistor devices used in the system have become extremely miniaturized. Accordingly, the size of each element of the semiconductor device is extremely miniaturized. In other words, a number of electronic elements mounted on the semiconductor device is tremendously increased as seen in an advanced semiconductor device such as an LSI (Large Scale Integration) device and a VLSI (Very Large Scale Integration) device. However, realizing a further reduction in size of the semiconductor device. In other words, realizing a higher packing density of the semiconductor device results in generating a large amount of heat from the semiconductor device. Accordingly, it has become impossible to keep an operating temperature of the semiconductor device in a maximum allowable in use, as long as a conventional air cooling method is used for cooling the semiconductor device. For example, the heating value per second of an LSI device is about 4 watts even in a maximum, and it increases up to as much as 10 watts in the case of a VLSI device. Therefore, to cool the semiconductor device, it has become necessary to employ a liquid cooling method in place of the conventional air cooling method.
Many kinds of liquid cooling units or structure have been practically used. For instance, FIG. 1 is a liquid cooling unit used for a flat package type semiconductor device, and FIG. 2 is liquid cooling structure used for a flip chip type semiconductor device. These liquid cooling unit and structure may be applied to any other types of semiconductor devices.
FIG. 1 indicates a mounting state of a liquid cooling unit 6 onto a flat package type semiconductor device 2 through an elastic heat conducing body 4 and a heat transferring plate 3 (made of, for example, alumina) equipped with the flat package type semiconductor device 2. The liquid cooling unit 6 comprises a cooling body 1, a bellows 5 made of metal or plastic, a heat conducting plate 9 connected to the bellows 5, a nozzle 7 and a water drain port 8. The heat conducting plate 9 is thermally connected with the heat transferring plate 3 through the elastic heat conducting body 4.
In FIG. 1, the nozzle 7 injects cooling water into a chamber formed by the bellows 5 for cooling the heat conducting plate 9 so that heat generated by the flat package type semiconductor device 2 is transferred to the cooling body 1, then the cooling water flows out from the water drain port 8 transferring heat from the heat conducting plate 9. Usually, the cooling temperature can be controlled by changing the temperature of the cooling water.
The elastic heat conducting body 4 is made of silicon rubber, in which a ceramic powder is mixed, for making the elastic heat conducting body 4 have an excellent heat conducting characteristic and good contact with both the heat conducting plate 9 and the heat transferring plate 3, using a pressure due to the elasticity of the bellows 5.
FIG. 2 indicates the structure for cooling a flip chip type semiconductor device 10 by utilizing a metal block (made of, for example, aluminum) 11 cooled by a cooling unit 13 which is also cooled by coolant flowing through a plurality of pipes 12 passing through the cooling unit 13. The cooing unit 13 has a recessed portion into which the metal block 11 is inserted pushing a coil spring 14. The metal block 11 has high heat conductivity and a smooth surface for making good contact with an inner wall surface of the recessed portion. The coil spring 14 is used to allow the metal block 11 to be placed in sufficient contact with the semiconductor device 10, with uniform pressure. The heat generated by the semiconductor device 10 is transferred to the cooling unit 13 through the metal block 11 The heat resistance appearing in gaps between the semiconductor device 10 and metal block 11 and between the metal block 11 and inner wall surface of the cooling unit 13 is reduced by using gas, such as helium, having good heat conduction, filled in the gaps and a space 20.
Many kinds of liquid cooling units have been used elsewhere, however considerably high heat resistance appears between the semiconductor device and the liquid cooing unit. Accordingly, in the prior art, the following methods have been proposed to lower the heat resistance:
1) depositing a soft metal (for example, indium or an indium alloy) into a contact portion intended to be thermally contacted, with pressure;
2) providing a liquid metal (for example, mercury) to the contact portion; and
3) soldering the contact portion.
However, in method 1), high thermal conductivity is difficult to achieve because the air layer always exists at a gap appearing in the contact portion. In method 2), there is always the danger of a short-circuit caused by flow of the liquid metal, because the liquid metal has low viscosity. In method 3), stress due to the difference in thermal expansion between the solder, the semiconductor device and the liquid cooling unit occurs, so that connecting structure around the contact portion is easily cracked when in operation and cooling is frequently performed.
Thus, the liquid cooling method is effective for cooling the semiconductor device, compared with the air cooling method. However, there is still a problem that a sufficient cooling effect is hard to be obtained because of large heat resistance appearing between the semiconductor device and the cooling unit, which has been a problem in the prior art.
A satellite flying in space is in a high vacuum, so that the temperature at the side of the satellite, facing the sun and that not facing the sun are quite different from each other. Therefore, making the temperature in the satellite uniform is very important for making components mounted in the satellite operate stably. As a result, the heat generated at the side facing the sun must be transferred to another side away from the sun. Furthermore, the components themselves generate heat respectively, so that such heat must be transferred to other places for keeping the temperatures of the components within allowable values. Usually, the heat of the components is transferred within the satellite and radiated into space, not directed toward the sun and the earth.
The satellite is fabricated by combining many structures, so that there are many fixed and rotatable mechanical joints in the construction of the structure. In these mechanical joints, the heat transfer which is carried out through these mechanical joints is very important, because the heat transferred through the structure is lost mostly at these mechanical joints. Therefore, how to reduce the heat transfer loss at the mechanical joints is a big problem in the manufacturing and operating of the satellite. To reduce the heat transfer loss at the mechanical joints, an organic material, such as silicon grease, optionally including metal and ceramic powder has been used. However, the organic material has the defect of being easily evaporated and changed in quality in a high vacuum so that the heat transfer loss of the material itself and at the contact to the mechanical structure increases.